DESCRIPTION (Applicant?s Abstract): The long-term objective of this project is to understand the molecular mechanism of meiosis and its regulation. Studying meiosis in metazoan oogenesis and the controls that couple meiosis to oocyte differentiation are important for understanding melosis in general. The first specific aim is to characterize the putative Drosophila mos homologue, a conserved kinase that functions during meiosis in vertebrates. Mos plays conserved and species-specific functions in Xenopus and mice. The Drosophila gene is the first invertebrate mos gene, providing an opportunity to determine its roles in fly meiosis. The expression pattern, and the effects of overexpression and deletion of Drosophila mos will be studied. This analysis may provide the first molecular insight about the metaphase arrest in flies. The second specific aim is to identify additional mutants that fail to complete meiosis. Previous such screens have been fruitful but not saturated. The primary screen will identify embryos with aberrant number of nuclei indicative of failure to complete meiosis and enter embryonic mitosis. Further characterization, including spindle morphology and onset of defect during oogenesis, will categorize the mutants based on the meiotic event that they affect. Mutants that affect the metaphase I arrest will be pursued further.